U.S. patent application number 12/078066 was filed with the patent office on 2008-11-27 for method for forming electrode for group-iii nitride compound semiconductor light-emitting devices.
This patent application is currently assigned to TOYODA GOSEI CO., LTD.. Invention is credited to Koichi Goshonoo, Miki Moriyama.
Application Number | 20080293231 12/078066 |
Document ID | / |
Family ID | 39915132 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080293231 |
Kind Code |
A1 |
Goshonoo; Koichi ; et
al. |
November 27, 2008 |
Method for forming electrode for Group-III nitride compound
semiconductor light-emitting devices
Abstract
A method for forming an electrode for Group-III nitride compound
semiconductor light-emitting devices includes a step of forming a
first electrode layer having an average thickness of less than 1 nm
on a Group-III nitride compound semiconductor layer, the first
electrode layer being made of a material having high adhesion to
the Group-III nitride compound semiconductor layer or low contact
resistance with the Group-III nitride compound semiconductor layer
and also includes a step of forming a second electrode layer made
of a highly reflective metal material on the first electrode
layer.
Inventors: |
Goshonoo; Koichi;
(Aichi-ken, JP) ; Moriyama; Miki; (Aichi-ken,
JP) |
Correspondence
Address: |
MCGINN INTELLECTUAL PROPERTY LAW GROUP, PLLC
8321 OLD COURTHOUSE ROAD, SUITE 200
VIENNA
VA
22182-3817
US
|
Assignee: |
TOYODA GOSEI CO., LTD.
Aichi-ken
JP
|
Family ID: |
39915132 |
Appl. No.: |
12/078066 |
Filed: |
March 26, 2008 |
Current U.S.
Class: |
438/605 ;
257/E21.172 |
Current CPC
Class: |
H01L 21/0254 20130101;
H01L 33/32 20130101; H01L 21/0262 20130101; H01L 33/405
20130101 |
Class at
Publication: |
438/605 ;
257/E21.172 |
International
Class: |
H01L 21/285 20060101
H01L021/285 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2007 |
JP |
2007-082810 |
Claims
1. A method for forming an electrode for Group-III nitride compound
semiconductor light-emitting devices, comprising: a step of forming
a first electrode layer having an average thickness of less than 1
nm on a Group-III nitride compound semiconductor layer, the first
electrode layer being made of a material having high adhesion to
the Group-III nitride compound semiconductor layer or low contact
resistance with the Group-III nitride compound semiconductor layer;
and a step of forming a second electrode layer made of a highly
reflective metal material on the first electrode layer.
2. The method according to claim 1, wherein the first electrode
layer is formed at a rate of 0.2 nm/s or less.
3. The method according to claim 1, wherein the material for
forming the first electrode layer is at least one selected from the
group consisting of metals such as titanium, vanadium, chromium,
nickel, indium, cobalt, copper, tungsten, tantalum, niobium, tin,
hafnium, zirconium, manganese, and magnesium; alloys of at least
two of the metals; alloys principally containing at least one of
the metals; nitrides of the metals; and carbides of the metals.
4. The method according to claim 2, wherein the material for
forming the first electrode layer is at least one selected from the
group consisting of metals such as titanium, vanadium, chromium,
nickel, indium, cobalt, copper, tungsten, tantalum, niobium, tin,
hafnium, zirconium, manganese, and magnesium; alloys of at least
two of the metals; alloys principally containing at least one of
the metals; nitrides of the metals; and carbides of the metals.
5. The method according to claim 1, wherein the highly reflective
metal material is at least one selected from the group consisting
of metals such as silver, aluminum, rhodium, and platinum; alloys
of at least two of the metals; and alloys principally containing at
least one of the metals and the second electrode layer has a
thickness of 0.03 to 5
6. The method according to claim 2, wherein the highly reflective
metal material is at least one selected from the group consisting
of metals such as silver, aluminum, rhodium, and platinum; alloys
of at least two of the metals; and alloys principally containing at
least one of the metals and the second electrode layer has a
thickness of 0.03 to 5 .mu.m.
7. The method according to claim 3, wherein the highly reflective
metal material is at least one selected from the group consisting
of metals such as silver, aluminum, rhodium, and platinum; alloys
of at least two of the metals; and alloys principally containing at
least one of the metals and the second electrode layer has a
thickness of 0.03 to 5 .mu.m.
8. The method according to claim 4, wherein the highly reflective
metal material is at least one selected from the group consisting
of metals such as silver, aluminum, rhodium, and platinum; alloys
of at least two of the metals; and alloys principally containing at
least one of the metals and the second electrode layer has a
thickness of 0.03 to 5.mu.m.
9. The method according to claim 1, wherein the electrode is an
n-electrode and the Group-III nitride compound semiconductor layer
is of a negative type.
10. The method according to claim 2, wherein the electrode is an
n-electrode and the Group-III nitride compound semiconductor layer
is of a negative type.
11. The method according to claim 3, wherein the electrode is an
n-electrode and the Group-III nitride compound semiconductor layer
is of a negative type.
12. The method according to claim 4, wherein the electrode is an
n-electrode and the Group-III nitride compound semiconductor layer
is of a negative type.
13. The method according to claim 5, wherein the electrode is an
n-electrode and the Group-III nitride compound semiconductor layer
is of a negative type.
14. The method according to claim 8, wherein the electrode is an
n-electrode and the Group-III nitride compound semiconductor layer
is of a negative type.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for forming an
electrode for Group-III nitride compound semiconductor
light-emitting devices. The term "Group-III nitride compound
semiconductor" used herein covers n- and p-type compound
semiconductors doped with any element; compound semiconductors
containing a Group-III or -V element and at least one selected from
the group consisting of B, Ti, P, As, Sb, and Bi; and compound
semiconductors represented by the formula
Al.sub.xGa.sub.yIn.sub.1-x-yN, wherein x, y, and x+y are zero to
one.
[0003] 2. Description of the Related Art
[0004] Conventional group-III nitride compound semiconductor
light-emitting devices have low light extraction efficiency. The
following documents disclose various techniques for forming
electrodes from metals having high visible light reflectivity:
Japanese Unexamined Patent Application Publication Nos.
2003-086843, 2004-179347, 2005-011857, and 2004-140052.
[0005] A conventional Group-III nitride compound semiconductor
light-emitting device (light-emitting diode) of a face up-type will
now be described. FIG. 8 is a sectional view of the Group-III
nitride compound semiconductor light-emitting device, which is
represented by reference numeral 900. The Group-III nitride
compound semiconductor light-emitting device 900 includes a
sapphire substrate 10, a buffer layer (not shown) made of aluminum
nitride (AlN), an n-type GaN layer 11 doped with Si, an n-type
AlGaN clad layer 12 doped with Si, a light-emitting layer 13 having
a GaN/InGaN multi-quantum well structure, a p-type AlGaN clad layer
14 doped with Mg, a p-type GaN layer 15 doped with Mg, and a
p.sup.+-type GaN layer 16, these layers being deposited on the
sapphire substrate 10 by metal-organic chemical vapor deposition
(MOCVD) or metal-organic physical vapor deposition (MOPVD) in that
order.
[0006] The p.sup.+-type GaN layer 16 is substantially covered with
a translucent electrode 21 made of indium tin oxide (ITO). The
translucent electrode 21 is partly covered with a pad electrode 22
made of gold. An n-electrode 90 including a vanadium (V) layer 91
and an aluminum (Al) layer 92 is disposed on the n-type GaN layer
11. Light is extracted through the translucent electrode 21; hence,
the Group-III nitride compound semiconductor light-emitting device
900 is a face up-type light-emitting diode (LED).
[0007] N-electrodes are usually made of aluminum, which is
inexpensive. Aluminum has high reflectivity for near-ultraviolet to
visible light and is suitable for electrodes for light-emitting
devices. However, the bonding strength between aluminum and gallium
nitride or the like is not necessarily high; hence, an aluminum
electrode should not be directly formed on a GaN layer but needs to
be formed on a contact electrode layer which is made of another
metal and which is disposed on the GaN layer.
[0008] With reference to FIG. 8, the vanadium layer 91 has a
thickness of about 20 nm (200 .ANG.). Investigations performed by
the present inventors have shown that the presence of the vanadium
layer 91, which has a thickness of about 20 nm (200 .ANG.), allows
the reflectivity of the interface between the n-electrode 90 and
the n-type GaN layer 11 to be about 40%, that is, the presence of
the vanadium layer 91 causes serious light absorption. In usual, an
n-electrode occupies about 10% of the area of a horizontal surface
of a light-emitting device. This n-electrode occupies such a large
area and has an absorptance of about 60%; hence, the influence of
this n-electrode on the light extraction efficiency of the
light-emitting device is non-negligible.
SUMMARY OF THE INVENTION
[0009] The present invention has been made to solve the above
problem. It is an object of the present invention to provide a
method for forming an electrode which has high reflectivity and
which is securely bondable to a gallium nitride layer.
[0010] According to a preferred embodiment of the present
invention, a method for forming an electrode for Group-III nitride
compound semiconductor light-emitting devices includes a step of
forming a first electrode layer having an average thickness of less
than 1 nm on a Group-III nitride compound semiconductor layer, the
first electrode layer being made of a material having high adhesion
to the Group-III nitride compound semiconductor layer or low
contact resistance with the Group-III nitride compound
semiconductor layer, and also includes a step of forming a second
electrode layer made of a highly reflective metal material on the
first electrode layer.
[0011] In the method, the first electrode layer is preferably
formed at a rate of 0.2 nm/s or less.
[0012] In the method, the material for forming the first electrode
layer is preferably at least one selected from the group consisting
of metals such as titanium (Ti), vanadium (V), chromium (Cr),
nickel (Ni), indium (In), cobalt (Co), copper (Cu), tungsten (W),
tantalum (Ta), niobium (Nb), tin (Sn), hafnium (Hf), zirconium
(Zr), manganese (Mn), and magnesium (Mg); alloys of at least two of
the metals; alloys principally containing at least one of the
metals; nitrides of the metals; and carbides of the metals.
[0013] In the method, the highly reflective metal material is
preferably at least one selected from the group consisting of
metals such as silver (Ag), aluminum (Al), rhodium (Rh), and
platinum (Pt); alloys of at least two of the metals; and alloys
principally containing at least one of the metals and the second
electrode layer preferably has a thickness of 0.03 to 5 .mu.m.
[0014] In the method, the electrode is preferably an n-electrode
and the Group-III nitride compound semiconductor layer is
preferably of a negative type.
[0015] In general, the bonding strength between a Group-III nitride
compound semiconductor layer and a highly reflective electrode made
of aluminum is insufficient. Therefore, a first electrode layer
made of titanium (Ti) or another material is provided on the
Group-III nitride compound semiconductor layer and a second
electrode layer made of a material such as aluminum (Al) is
provided on the first electrode layer. The first electrode layer
has a thickness of less than 1 nm. In order to control the first
electrode layer to be extremely thin, the first electrode layer is
preferably formed at a rate of 0.2 nm/s (2 .ANG./s) or less in the
thickness direction thereof.
[0016] Since the first electrode layer is very thin, the first
electrode layer has no smooth surface but an irregular surface. The
first electrode layer preferably has surface irregularities having
a height equal to one half of the average thickness thereof to the
average thickness thereof. This leads to an increase in the bonding
strength between the first electrode layer and the Group-III
nitride compound semiconductor layer and an increase in the contact
area between the first and second electrode layers and allows the
first electrode layer to have an extremely small thickness. When
the first electrode layer has an extremely small thickness, the
light absorption of the first electrode layer can be sufficiently
suppressed and the bonding strength between the Group-III nitride
compound semiconductor layer and the electrode including the first
and second electrode layers can be maintained high. Therefore, the
following device can be achieved: a Group-III nitride compound
semiconductor light-emitting device which has high light extraction
efficiency and which includes an electrode having high bonding
strength.
[0017] The bonding strength of the first electrode layer, which has
a thickness of less than 1 nm, is equal to about three fourths and
is not less than one half of that of an electrode layer with a
thickness of about 20 nm (200 .ANG.). This causes no problem in
practical use.
[0018] A method according to the present invention can be used to
form an electrode on an n- or p-type Group-III nitride compound
semiconductor layer. This electrode may be of a positive type.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1A is a plan view of a structure including a first
electrode and second electrode used to measure contact
resistance;
[0020] FIG. 1B is a sectional view of the structure shown in FIG.
1A;
[0021] FIG. 1C is a graph showing the current-voltage curve of the
structure shown in FIG. 1A and that of a comparative structure;
[0022] FIG. 2A is a graph showing the reflectivity of a first
sample of Example 1, that of a second sample of Comparative Example
1, and that of a third sample for comparison;
[0023] FIG. 2B is a graph showing the reflectivity of a fourth
sample of Example 2 and that of a fifth sample of Comparative
Example 2;
[0024] FIG. 3 is a sectional view of a Group-III nitride compound
semiconductor light-emitting device according to an embodiment of
the present invention;
[0025] FIG. 4 is an illustration showing the AMF profile of a 16
.ANG. thick titanium layer and that of an 8 .ANG. thick titanium
layer;
[0026] FIG. 5 is a sectional view of a Group-III nitride compound
semiconductor light-emitting device according to another embodiment
of the present invention;
[0027] FIG. 6 is a sectional view of a Group-III nitride compound
semiconductor light-emitting device according to another embodiment
of the present invention;
[0028] FIG. 7 is a sectional view of a Group-III nitride compound
semiconductor light-emitting device according to another embodiment
of the present invention; and
[0029] FIG. 8 is a sectional view of a conventional Group-III
nitride compound semiconductor light-emitting device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] A first electrode layer and a second electrode layer can be
formed by sputtering, vapor deposition, or another known process.
The first electrode layer has an extremely small thickness.
Therefore, it is necessary to control the average thickness of the
first electrode layer.
[0031] The first electrode layer has surface irregularities. The
surface irregularities preferably project from an imaginary plane
of the first electrode layer at 5 .ANG. or more. The first
electrode layer preferably has an average thickness of not less
than 0.3 nm (3 .ANG.).
[0032] The second electrode layer preferably has a thickness of
0.03 to 5 .mu.m. When the second electrode layer has a thickness of
less than 0.03 .mu.m, the reflectivity of the second electrode
layer is insufficient. There is no advantage if the second
electrode layer has a thickness of greater than 5 .mu.m. If the
second electrode layer needs to be formed from aluminum, which is
inexpensive, so as to be thick, the second electrode layer may have
a desired thickness. When the thickness of the second electrode
layer is 0.1 to 2.5 .mu.m, the second electrode layer has a
sufficient reflectivity.
[0033] In view of device assembly and the like, another electrode
layer may be deposited on the second electrode layer.
[0034] The first and second electrode layers are preferably formed
in such a manner that, for example, a photoresist mask is formed so
as to cover a region that is not used to form the first and second
electrode layers and unnecessary portions are removed from the
first and second electrode layers by lifting off the photoresist
mask after the first and second electrode layers are formed. The
first and second electrode layers are preferably thermally treated,
or annealed. The thermal treatment temperature of the first and
second electrode layers may be 100.degree. C. to 650.degree. C.
[0035] According to the present invention, a Group-III nitride
compound semiconductor light-emitting device can be readily
manufactured. Examples of the Group-III nitride compound
semiconductor light-emitting device include light-emitting diode
(LEDs), laser diodes (LDs), photocouplers, and other light-emitting
devices. Any method can be used to manufacture the Group-III
nitride compound semiconductor light-emitting device.
[0036] A substrate for crystal growth may be made of sapphire,
spinel, Si, SiC, ZnO, MgO, or a single-crystalline Group-III
nitride-based compound. The following technique is effective in
forming a Group-III nitride compound semiconductor layer: molecular
beam epitaxy (MBE), metal-organic vapor-phase epitaxy (MOVPE),
halide vapor-phase epitaxy (HVPE), or the like.
[0037] In order to form an n-type Group-III nitride compound
semiconductor layer negative, an n-type impurity such as Si, Ge,
Se, Te, or C may be used. In order to form a p-type Group-III
nitride compound semiconductor layer positive, a p-type impurity
such as Zn, Mg, Be, Ca, Sr, or Ba may be used.
[0038] A light-emitting layer may have a single-layer structure, a
single-quantum well (SQW) structure, a multi-quantum well (MQW)
structure, or another structure.
[0039] The second electrode layer has high reflectivity and
therefore is made of a material having high reflectivity for green
light, blue light, and near-ultraviolet light. The second electrode
layer is preferably made of a single metal such as aluminum,
rhodium, platinum, or silver. Alternatively, the second electrode
layer may be made of an alloy principally containing such a metal
or may have a multilayer structure containing such a metal.
EXAMPLES
[0040] Advantages of the present invention will now be described
with reference to experiments performed by the present
inventors.
Example 1
[0041] Electrodes according to the present invention were measured
for contact resistance with respect to n-type GaN as described
below.
[0042] FIG. 1A is a plan view of a structure including a first
electrode C and second electrode R measured for contact resistance.
FIG. 1B is a sectional view of the structure. As shown in FIG. 1B,
a buffer layer (not shown) made of aluminum nitride (AlN) was
formed on a sapphire substrate 10. A layer of GaN was formed on the
buffer layer by MOVPE so as to have a thickness of about 4 .mu.m
and then doped with Si at a dose of 4.times.10.sup.18 cm.sup.-3,
whereby an n-type GaN layer 1 was formed. The first and second
electrodes C and R were formed on the n-type GaN layer 1 by vacuum
vapor deposition. The first and second electrodes C and R may be
formed by a sputtering process.
[0043] As shown in FIG. 1A, the first electrode C was disk-shaped
and had a diameter of 400 .mu.m. The second electrode R was spaced
from the first electrode C at a distance of 24 .mu.m and was
ring-shaped.
[0044] The first and second electrodes C and R each included a
titanium (Ti) layer with a thickness of 0.5 nm (5 .ANG.) and an
aluminum (Al) layer with a thickness of 2 .mu.m (Example 1).
[0045] A comparative structure was prepared (Comparative Example
1). The comparative structure had substantially the same
configuration as that of the structure of Example 1 except that the
comparative structure included a third electrode and fourth
electrode each including a vanadium (V) layer with a thickness of
17.5 nm (175 .ANG.) and an aluminum (Al) layer with a thickness of
0.2 .mu.m.
[0046] FIG. 1C is a graph showing the current-voltage curve of the
structure thermally treated at 570.degree. C. and that of the
comparative structure thermally treated at 570.degree. C. The
current-voltage curves are linear. This shows that the ohmic
electrodes suitable for n-type GaN can be obtained. The current
flowing through the structure (represented by .DELTA.) is greater
than that flowing through the comparative structure (represented by
.largecircle.) although the same voltage is applied to the
structure and the comparative structure. This shows that the
contact resistance of the first and second electrodes C and R with
respect to the n-type GaN layer 1 is less than that of the third
and fourth electrodes with respect to an n-type GaN layer included
in the comparative structure.
[0047] The contact resistance of the first and second electrodes C
and R with respect to the n-type GaN layer 1 was determined to be
1.9.times.10.sup.-5 .OMEGA./cm.sup.2 by a TLM method in such a
manner that the distance between the first and second electrodes C
and R was varied. The contact resistance of the third and fourth
electrodes with respect to the n-type GaN layer of the comparative
structure was determined to be 6.5.times.10.sup.-5 .OMEGA./cm.sup.2
in the same manner as described above. This shows that according to
the present invention, an n-electrode having a contact resistance
less than one third of that of a conventional n-electrode can be
provided.
Example 2
[0048] An electrode according to the present invention was
evaluated for reflectivity by a method below.
[0049] A first sample including a first sapphire substrate with a
thickness of 400 .mu.m and a first electrode disposed thereon was
prepared in such a manner that a titanium (Ti) layer with a
thickness of 0.5 nm (5 .ANG.) and a first aluminum (Al) layer with
a thickness of 2 .mu.m were deposited on the first sapphire
substrate in that order (Example 2).
[0050] A second sample, including a second sapphire substrate with
a thickness of 400 .mu.m and a second electrode disposed thereon,
for comparison was prepared in such a manner that a first vanadium
(V) layer with a thickness of 17.5 nm (175 .ANG.) and a second
aluminum (Al) layer with a thickness of 2 .mu.m were deposited on
the second sapphire substrate in that order (Comparative Example
2).
[0051] A third sample, including a third sapphire substrate and a
third electrode disposed thereon, for comparison was prepared in
such a manner that a third aluminum (Al) layer with a thickness of
2 .mu.m was formed on the third sapphire substrate by vapor
deposition.
[0052] The first, second, and third electrodes were measured for
reflectivity in such a manner that light with a wavelength of 350
to 600 nm was applied to the first, second, and third sapphire
substrates, respectively. The obtained measurements are not equal
to the reflectivities of the first, second, and third electrodes in
the strict sense but can be used to evaluate the first, second, and
third electrodes for reflectivity because the reflectivity of the
interface between air and each of the first, second, and third
sapphire substrates is sufficiently small.
[0053] The first and second samples were measured for reflectivity
before and after the first and second samples were annealed at
570.degree. C. The evaluation results are summarized in FIG.
2A.
[0054] The followings are clear from FIG. 2A.
[0055] The third sample has a reflectivity of 87% to 89% at a
wavelength of 350 to 600 nm.
[0056] The thermally untreated first sample has a reflectivity of
82% to 84% at a wavelength of 350 to 600 nm. The thermally treated
first sample has a reflectivity of 85% to 86% at a wavelength of
350 to 600 nm. That is, the reflectivity of the first sample was
hardly reduced but was slightly increased by thermal treatment.
This is probably because the titanium layer and the first aluminum
layer were alloyed with each other by thermal treatment.
[0057] The thermally untreated second sample has a low reflectivity
of 38% to 48% at a wavelength of 350 to 600 nm. The thermally
treated second sample has a reflectivity of 55% to 60% at a
wavelength of 350 to 600 nm. This is probably because the first
vanadium layer and the second aluminum layer were alloyed with each
other by thermal treatment and therefore the reflectivity of the
second sample was increased. The reflectivity of the thermally
treated second sample is far less than that of the thermally
treated first sample.
[0058] As is clear from FIG. 2A, according to the present
invention, the titanium layer, which is effective in bonding an
n-type GaN layer and a highly reflective metal layer such as an
aluminum layer together, has an extremely small thickness of less
than 1 nm (1 .ANG.) and therefore has low absorptance; hence, the
first electrode, which is an n-electrode, has high reflectivity.
Even if the titanium layer and the first aluminum layer are not
thermally treated for alloying or are thermally treated at low
temperature and therefore are not alloyed with each other, the
first electrode has high reflectivity.
[0059] Structures were prepared in such a manner that the following
layers were deposited on each epitaxial substrate by MOCVD (MOPVD)
in this order: a buffer layer made of aluminum nitride (AlN), an
n-type GaN layer 11 doped with Si, an n-type AlGaN clad layer 12
doped with Si, a light-emitting layer 13 having a GaN/InGaN
multi-quantum well structure, a p-type AlGaN clad layer 14 doped
with Mg, a p-type GaN layer 15 doped with Mg, and a p.sup.+-type
GaN layer 16. The epitaxial substrate was substantially the same as
the sapphire substrate 10 of the Group-III nitride compound
semiconductor light-emitting device 900 shown in FIG. 8. A fourth
sample (Example 2) was prepared in such a manner that a second
titanium (Ti) layer with a thickness of 0.5 nm (5 .ANG.) and a
fourth aluminum (Al) layer with a thickness of 2 .mu.m were
deposited on the p.sup.+-type GaN layer 16 of one of the structures
in that order such that a third electrode was formed on the
p.sup.+-type GaN layer 16. A fifth sample (Comparative Example 2)
was prepared in such a manner that a second vanadium (V) layer with
a thickness of 17.5 nm (175 .ANG.) and a fifth aluminum (Al) layer
with a thickness of 2 .mu.m were deposited on the p.sup.+-type GaN
layer 16 of one of the structures in that order such that a fourth
electrode was formed on the p.sup.+-type GaN layer 16. The third
and fourth electrodes were measured for reflectivity in the same
manner as that described above. The measurement results are shown
in FIG. 2B. FIG. 2B illustrates that the reflectivity of each of
the third and fourth electrodes varies depending on the wavelength
of light because epitaxial layers absorb light with a wavelength of
400 nm or less and interference occurs depending on the thickness
of the epitaxial layers and the wavelength of light.
[0060] The followings are clear from FIG. 2B.
[0061] The third electrode of the thermally treated fourth sample
of Example 2 has a reflectivity of 50% or more at a wavelength of
380 nm or more and a reflectivity of 70% or more at a wavelength of
430 nm or more. The reflectivity of the third electrode slightly
varies depending on the wavelength of light and is about 75% (71%
to 82%).
[0062] The fourth electrode of the thermally treated fifth sample
of Comparative Example 2 has a reflectivity of not greater than 46%
at a wavelength of 380 to 600 nm. Significant interference is
caused in the thermally treated fifth sample because of the
thickness of the epitaxial layers. That is, the reflectivity of the
third electrode of the thermally treated fourth sample is 1.8 to
2.4 times that of the fourth electrode of the thermally treated
fifth sample. Therefore, according to the present invention, a
highly reflective electrode can be obtained.
Example 3
[0063] Light-emitting devices were prepared and then evaluated as
described below.
[0064] FIG. 3 is a sectional view of a first Group-III nitride
compound semiconductor light-emitting device 100 according to an
embodiment of the present invention. The first Group-III nitride
compound semiconductor light-emitting device 100 includes a
sapphire substrate 10, a buffer layer (not shown) made of aluminum
nitride (AlN), an n-type GaN layer 11 doped with Si, an n-type
AlGaN clad layer 12 doped with Si, a light-emitting layer 13 having
a GaN/InGaN multi-quantum well structure, a p-type AlGaN clad layer
14 doped with Mg, a p-type GaN layer 15 doped with Mg, and a
p.sup.+-type GaN layer 16, these layers being deposited on the
sapphire substrate 10 by MOCVD or MOPVD in that order.
[0065] The first Group-III nitride compound semiconductor
light-emitting device 100 further includes a translucent electrode
21, made of ITO, extending over the p.sup.+-type GaN layer 16; a
pad electrode 22, made of gold, lying on a portion of the
translucent electrode 21; and an n-electrode 30 including a Ti
layer 31 and an Al layer 32. The n-electrode 30 reflects light and
light is extracted through the translucent electrode 21; hence, the
first Group-III nitride compound semiconductor light-emitting
device 100 is a face up-type light-emitting diode.
[0066] The first Group-III nitride compound semiconductor
light-emitting device 100 was manufactured by a method below.
Gaseous materials used were as follows: ammonia (NH.sub.3),
hydrogen (H.sub.2) as a carrier gas, nitrogen (N.sub.2) as a
carrier gas, trimethyl gallium (TMG), trimethyl aluminum (TMA),
trimethyl indium (TMI), silane (SiH.sub.4), and
bis(cyclopentadienyl)magnesium (Cp.sub.2Mg).
[0067] The sapphire substrate 10 was single-crystalline and had a
principal surface organically and thermally cleaned. The sapphire
substrate 10 was attached to a susceptor placed in a reaction
chamber of a MOCVD system. The sapphire substrate 10 was baked at
1,100.degree. C. for about 30 minutes while the reaction chamber
was being supplied with H.sub.2 at a flow rate of 2 L/min at normal
pressure.
[0068] After the temperature of the sapphire substrate 10 was
reduced to 400.degree. C., the reaction chamber was supplied with
H.sub.2, NH.sub.3, and TMA for about one minute at a H.sub.2 flow
rate of 20 L/min, an NH.sub.3 flow rate of 10 L/min, and a TMA flow
rate of 1.8.times.10.sup.-5 mol/min, whereby the buffer layer was
formed on the principal surface of the sapphire substrate 10 so as
to have a thickness of about 25 nm.
[0069] After the temperature of the sapphire substrate 10 was
increased to 1,150.degree. C., the reaction chamber was supplied
with H.sub.2, NH.sub.3, TMG, and a SiH.sub.4--H.sub.2 gas mixture
having a SiH.sub.4 content of 0.86 ppm for about 40 minutes at a
H.sub.2 flow rate of 20 L/min, an NH.sub.3 flow rate of 10 L/min, a
TMG flow rate of 1.78.times.10.sup.-4 mol/min, and a SiH.sub.4 flow
rate of 20.times.10.sup.-8 mol/min, respectively, whereby the
n-type GaN layer 11 was formed on the buffer layer so as to have a
thickness of about 4.0 .mu.m, an electron concentration of
2.times.10.sup.18 cm.sup.-3, and a silicon concentration of
4.times.10.sup.18 cm.sup.-3.
[0070] The reaction chamber was supplied with NH.sub.3, TMG, TMA, a
SiH.sub.4--H.sub.2 gas mixture having a SiH.sub.4 content of 0.86
ppm, and N.sub.2 or H.sub.2 for about 60 minutes at an NH.sub.3
flow rate of 10 L/min, a TMG flow rate of 1.12.times.10.sup.-4
mol/min, a TMA flow rate of 0.47.times.10.sup.-4 mol/min, a
SiH.sub.4 flow rate of 5.times.10.sup.-9 mol/min, and a N.sub.2 or
H.sub.2 flow rate of 10 L/min, respectively, while the sapphire
substrate 10 was maintained at 1,150.degree. C., whereby the n-type
AlGaN clad layer 12 was formed on the n-type GaN layer 11 so as to
have a thickness of about 0.5 .mu.m, an electron concentration of
1.times.10.sup.18 cm.sup.-3, and a silicon concentration of
2.times.10.sup.18 cm.sup.-3. The n-type AlGaN clad layer 12 had the
formula Al.sub.0.08Ga.sub.0.92N.
[0071] After the n-type AlGaN clad layer 12 was formed, the
reaction chamber was supplied with NH.sub.3, TMG, and N.sub.2 or
H.sub.2 for about one minute at an NH.sub.3 flow rate of 10 L/min,
a TMG flow rate of 2.0.times.10.sup.-4 mol/min, and a N.sub.2 or
H.sub.2 flow rate of 20 L/min, respectively, whereby a GaN barrier
sublayer having a thickness of about 35 .ANG. was formed on the
n-type AlGaN clad layer 12. The reaction chamber was supplied with
NH.sub.3, TMG, TMI, and N.sub.2 or H.sub.2 for about one minute at
an NH.sub.3 flow rate of 10 L/min, a TMG flow rate of
7.2.times.10.sup.-5 mol/min, a TMI flow rate of
0.19.times.10.sup.-4 mol/min, and a N.sub.2 or H.sub.2 flow rate of
20 L/min, respectively, whereby a well sublayer which had a
thickness of about 35 .ANG. and which had the formula
In.sub.0.20Ga.sub.0.80N was formed on the GaN barrier sublayer.
This procedure was repeated five times. Another GaN barrier
sublayer was formed on the uppermost well sublayer under the same
conditions as described above, whereby the light-emitting layer 13
was formed. The light-emitting layer 13 had a five-period
multi-quantum well structure.
[0072] The reaction chamber was supplied with NH.sub.3, TMG, TMA,
Cp.sub.2Mg, and N.sub.2 or H.sub.2 for about three minutes at an
NH.sub.3 flow rate of 10 L/min, a TMG flow rate of
1.0.times.10.sup.-4 mol/min, a TMA flow rate of 1.0.times.10.sup.-4
mol/min, a Cp.sub.2Mg flow rate of 2.times.10.sup.-5 mol/min, and a
N.sub.2 or H.sub.2 flow rate of 10 L/min, respectively, while the
sapphire substrate 10 was maintained at 1,100.degree. C., whereby
the p-type AlGaN clad layer 14 doped with Mg was formed on the
light-emitting layer 13. The p-type AlGaN clad layer 14 had a
thickness of about 50 nm, a magnesium concentration of
5.times.10.sup.19 cm.sup.-3, and the formula
Al.sub.0.15Ga.sub.0.85N.
[0073] The reaction chamber was supplied with NH.sub.3, TMG,
Cp.sub.2Mg, and N.sub.2 or H.sub.2 for about 30 seconds at an
NH.sub.3 flow rate of 10 L/min, a TMG flow rate of
1.12.times.10.sup.-4 mol/min, a Cp.sub.2Mg flow rate of
2.times.10.sup.-5 mol/min, and a N.sub.2 or H.sub.2 flow rate of 20
L/min, respectively, while the sapphire substrate 10 was maintained
at 1,100.degree. C., whereby the p-type GaN layer 15 doped with Mg
was formed on the p-type AlGaN clad layer 14. The p-type GaN layer
15 had a thickness of about 100 nm and a magnesium concentration of
5.times.10.sup.19 cm.sup.-3. The p.sup.+-type GaN layer 16 doped
with magnesium was formed on the p-type GaN layer 15 so as to have
a thickness of about 10 nm and a magnesium concentration of
1.times.10.sup.20 cm.sup.-3.
[0074] A photoresist etching mask was provided on the p.sup.+-type
GaN layer 16 and predetermined regions were then removed from the
photoresist etching mask. The following portion and layers were
partly removed by reactive ion etching using a reactive gas
containing chlorine: a portion of the p.sup.+-type GaN layer 16
that was uncovered from the photoresist etching mask, the p-type
GaN layer 15, the p-type AlGaN clad layer 14, the light-emitting
layer 13, the n-type AlGaN clad layer 12, and the n-type GaN layer
11. This allowed the n-type GaN layer 11 to be exposed. An ITO
layer was formed on the p.sup.+-type GaN layer 16 by vacuum vapor
deposition so as to have a thickness of 300 nm and then thermally
treated at 700.degree. C. in a nitrogen atmosphere. An unnecessary
portion was removed from the ITO layer by wet etching using a
photoresist mask, whereby the translucent electrode 21 was
formed.
[0075] A photoresist mask having windows corresponding to necessary
portions was formed over the n-type GaN layer 11 and the
translucent electrode 21. Electrode materials were provided over
this photoresist mask, which was then lifted off. Unnecessary
portions were removed from the electrode materials, whereby
electrodes below were formed.
[0076] In particular, the pad electrode 22 was formed on the
translucent electrode 21 using gold. The Ti layer 31, or a first
electrode layer, was formed on the n-type GaN layer 11 at a rate of
6.0 nm/min (60 .ANG./min) by vacuum vapor deposition so as to have
a thickness of 0.5 nm (5 .ANG.). The Al layer 32, or a second
electrode layer, was then formed on the Ti layer 31, whereby the
n-electrode 30 was formed. The n-electrode 30 was thermally treated
at 570.degree. C. for five minutes, whereby the Ti layer 31 and the
Al layer 32 were alloyed with each other.
[0077] A second Group-III nitride compound semiconductor
light-emitting device 900 for comparison was prepared as shown in
FIG. 8 (Comparative Example 2). The second Group-III nitride
compound semiconductor light-emitting device 900 had substantially
the same configuration as that of the first Group-III nitride
compound semiconductor light-emitting device 100 except that the
second Group-III nitride compound semiconductor light-emitting
device 900 included a vanadium (V) layer 91 with a thickness of
17.5 nm (175 .ANG.) instead of the Ti layer 31. The first and
second Group-III nitride compound semiconductor light-emitting
devices 100 and 900 had a size of 240 .mu.m.times.480 .mu.m in plan
view. The n-electrodes 30 had an area, in plan view, equal to about
10% of that of the first and second Group-III nitride compound
semiconductor light-emitting devices 100 and 900.
[0078] Comparisons between properties of the first and second
Group-III nitride compound semiconductor light-emitting devices 100
and 900 were as described below.
[0079] The radiant flux of the first Group-III nitride compound
semiconductor light-emitting device 100 was 1.07 times that of the
second Group-III nitride compound semiconductor light-emitting
device 900, that is, the light extraction efficiency of the first
Group-III nitride compound semiconductor light-emitting device 100
was seven percent higher than that of the second Group-III nitride
compound semiconductor light-emitting device 900.
[0080] The driving voltage of the first Group-III nitride compound
semiconductor light-emitting device 100 was less than or equal to
that of the second Group-III nitride compound semiconductor
light-emitting device 900 when the first and second Group-III
nitride compound semiconductor light-emitting devices 100 and 900
were supplied with a current of 20 mA.
[0081] The first and second Group-III nitride compound
semiconductor light-emitting devices 100 and 900 were subjected to
a peel test (a shear strength test) in such a manner that wires
were bonded to the n-electrodes 30 and 90 and the shear rate was
200 .mu.m/s. The shear strength of the first Group-III nitride
compound semiconductor light-emitting device 100 was 0.76 times
that of the second Group-III nitride compound semiconductor
light-emitting device 900, that is, the shear strength of the first
Group-III nitride compound semiconductor light-emitting device 100
was slightly less than that of the second Group-III nitride
compound semiconductor light-emitting device 900. However, the
shear strength of the first Group-III nitride compound
semiconductor light-emitting device 100 was greater than 0.4 times
that of the second Group-III nitride compound semiconductor
light-emitting device 900; hence, the first Group-III nitride
compound semiconductor light-emitting device 100 was sufficient for
practical use.
Example 4
[0082] In order to investigate the reason why the strength was not
reduced in the peel test described in Example 3, thin titanium
layers were analyzed for thickness distribution by atomic force
microscopy (AFM) as described below. A 16 .ANG. thick titanium
layer was formed on a first sapphire substrate in such a manner
that titanium was deposited on the first sapphire substrate for 40
seconds at a rate of 2.4 nm/min (24 .ANG./min). An 8 .ANG. thick
titanium layer was formed on a second sapphire substrate in such a
manner that titanium was deposited on the second sapphire substrate
for 20 seconds at a rate of 2.4 nm/min (24 .ANG./min). The 16 .ANG.
and 8 .ANG. thick titanium layers were analyzed for thickness
distribution by AFM. The analysis results are summarized in FIG. 4.
The AMF profile of the 16 .ANG. thick titanium layer is
significantly different from that of the 8 .ANG. thick titanium
layer.
[0083] With reference to FIG. 4, the 16 .ANG. thick titanium layer
has a smooth surface, that is, the 16 .ANG. thick titanium layer
has small surface irregularities. In the AFM profile of the 16
.ANG. thick titanium layer, the vertical distance between the top
of the highest peak and the bottom (located at a 0 .ANG. position
in FIG. 4) of the deepest valley is 6 .ANG.. Suppose that the
height of an imaginary plane for defining the average thickness of
the 16 .ANG. thick titanium layer from the bottom of the deepest
valley is about 2.5 .ANG., the top of the highest peak is only
about 3.5 .ANG. higher than the imaginary plane. The AFM profile of
the 16 .ANG. thick titanium layer has only three peaks that are
higher than the imaginary plane. The other peaks are lower than the
imaginary plane.
[0084] In contrast, the 8 .ANG. thick titanium layer has a rough
surface, that is, the 8 .ANG. thick titanium layer has large
surface irregularities. In the AFM profile of the 8 .ANG. thick
titanium layer, the vertical distance between the top of the
highest peak and the bottom (located at a 0 .ANG. position in FIG.
4) of the deepest valley is 10 .ANG.. Suppose that the height of an
imaginary plane for defining the average thickness of the 8 .ANG.
thick titanium layer from the bottom of the deepest valley is about
4 .ANG., the tops of three peaks are about 6 .ANG. higher than the
imaginary plane. The AFM profile of the 8 .ANG. thick titanium
layer has five peaks that are higher than the imaginary plane.
Other peaks are also high.
[0085] As shown in FIG. 4, the AFM profile of the 16 .ANG. thick
titanium layer, which is thicker than 1 nm (10 .ANG.), is
significantly different from that of the 8 .ANG. thick titanium
layer, which is thinner than 1 nm (10 .ANG.). The surface
irregularities of the 8 .ANG. thick titanium layer are large as
described above. Therefore, the 8 .ANG. thick titanium layer has a
large contact area with an upper layer disposed thereon and the
anchoring effect due to the surface irregularities probably
enhances the adhesion between the 8 .ANG. thick titanium layer and
the upper layer. The ratio of the surface area to the volume of the
8 .ANG. thick titanium layer, which is extremely thin, is large;
hence, the strain on the 8 .ANG. thick titanium layer can be
readily reduced because of the diffusion of atoms present at the
surface of the 8 .ANG. thick titanium layer. A layer of a material
such as tungsten has a large intrinsic strain (or stress) and
therefore it is difficult to bond the material layer to another
layer when the material layer has a large thickness. However, if
the material layer is formed by a method according to the present
invention so as to have an extremely small thickness, the material
layer probably has a small intrinsic strain and therefore has
sufficient adhesion to another layer. Even the following material
can be used to form a first electrode layer if the first electrode
layer is formed by the method according to the present invention so
as to have an extremely small thickness: a material which has low
contact resistance with a Group-III nitride compound semiconductor
layer and of which a layer has low adhesion to another layer if the
layer is formed by a conventional method. These advantages probably
allow the first electrode layer, which has an extremely small
thickness, to have sufficient bonding strength.
Example 5
[0086] FIG. 5 is sectional view of a Group-III nitride compound
semiconductor light-emitting device 200 according to an embodiment
of the present invention. The Group-III nitride compound
semiconductor light-emitting device 200 is different from the first
Group-III nitride compound semiconductor light-emitting device 100,
manufactured in Example 3, shown in FIG. 3 in that a highly
reflective electrode 25 made of rhodium (Rh) and a pad electrode 26
are arranged on a p.sup.+-type GaN layer 16 in that order and the
thickness of the highly reflective electrode 25 and that of the pad
electrode 26 are adjusted such that the top of the pad electrode 26
is substantially flush with the top (a lower position in FIG. 5) of
a second electrode layer 32 which is made of aluminum (Al) and
which is an n-electrode. Other components of the Group-III nitride
compound semiconductor light-emitting device 200 can be readily
formed by a known technique similar to the method for manufacturing
the first Group-III nitride compound semiconductor light-emitting
device 100. The Group-III nitride compound semiconductor
light-emitting device 200 is a flip chip-type light-emitting diode
in which an n-electrode and a p-electrode have high reflectivity
and light is extracted through the rear surface of a sapphire
substrate 10 (in the upper direction in FIG. 5). The use of the
n-electrode according to the present invention leads to an increase
in light extraction efficiency.
Example 6
[0087] FIG. 6 is sectional view of a Group-III nitride compound
semiconductor light-emitting device 300 according to an embodiment
of the present invention. The Group-III nitride compound
semiconductor light-emitting device 300 is different from the first
Group-III nitride compound semiconductor light-emitting device 100,
manufactured in Example 3, shown in FIG. 3 in that a conductive
n-type GaN substrate 110 is used and an n-electrode is disposed on
the rear surface of the conductive n-type GaN substrate 110. Other
components of the Group-III nitride compound semiconductor
light-emitting device 300 can be readily formed by a known
technique similar to the method for manufacturing the first
Group-III nitride compound semiconductor light-emitting device 100.
The Group-III nitride compound semiconductor light-emitting device
300 is a face up-type vertical light-emitting diode including a
p-type electrode located at an upper position and an n-type
electrode located at a lower position. The n-type electrode has
high resistivity. The use of the n-type electrode leads to an
increase in light extraction efficiency. In a flip chip-type
light-emitting diode that includes a conductive n-type GaN
substrate 110, a p-type electrode which is made of rhodium (Rh) and
which has high reflectivity, and an n-type pad electrode, disposed
on a principal surface of the conductive n-type GaN substrate 110,
for wire bonding, the use of an n-type electrode which is formed by
a method according to the present invention so as to have high
reflectivity leads to an increase in light extraction
efficiency.
Example 7
[0088] FIG. 7 is sectional view of a Group-III nitride compound
semiconductor light-emitting device 400 according to an embodiment
of the present invention. The Group-III nitride compound
semiconductor light-emitting device 400 is different from the first
Group-III nitride compound semiconductor light-emitting device 100,
manufactured in Example 3, shown in FIG. 3 in that an n.sup.+-type
GaN layer 19 with a high donor concentration is provided on a
p.sup.+-type GaN layer 16 and a first electrode layer 31' and a
second electrode layer 32' are formed on the n.sup.+-type GaN layer
19 in that order by a method according to the present invention.
The first and second electrode layers 31' and 32' cooperatively
function as a positive electrode for the Group-III nitride compound
semiconductor light-emitting device 400. Electrons can be injected
into the interface between the n.sup.+-type GaN layer 19 and the
p.sup.+-type GaN layer 16 by tunnel transport. Other components of
the Group-III nitride compound semiconductor light-emitting device
400 can be readily formed by a known technique similar to the
method for manufacturing the first Group-III nitride compound
semiconductor light-emitting device 100. The Group-III nitride
compound semiconductor light-emitting device 400 is a flip
chip-type light-emitting diode in which a positive electrode and a
negative electrode are formed by a method according to the present
invention so as to have high reflectivity and light is extracted
through the rear surface of a sapphire substrate 10 (in the upper
direction in FIG. 5). The use of an n-electrode according to the
present invention for the positive and negative electrodes leads to
an increase in light extraction efficiency. Since the positive and
negative electrodes can be simultaneously formed, the Group-III
nitride compound semiconductor light-emitting device 400 can be
manufactured by a simple process at low cost.
[0089] In Example 3, the light-emitting layer 13 of the first
Group-III nitride compound semiconductor light-emitting device 100
has such a multi-quantum well structure and may have a
single-quantum well structure or an In.sub.0.2Ga.sub.0.8N
single-layer structure or may contain a ternary or quaternary
eutectic such as AlInGaN. Mg is used as a p-type impurity and a
Group-II (Group-IIa or -IIb) element such as beryllium (Be) or zinc
(Zn) may be used instead of Mg.
[0090] In Example 3, the Ti layer 31, which serves as a first
electrode layer and has a thickness of less than 1 nm, is disposed
on the n-type GaN layer 11. A first electrode layer, made of the
following material, having a thickness of less than 1 nm may be
disposed on a Group-III nitride compound semiconductor layer having
an arbitrary composition: a metal such as titanium (Ti), vanadium
(V), chromium (Cr), nickel (Ni), indium (In), cobalt (Co), copper
(Cu), tungsten (W), tantalum (Ta), niobium (Nb), tin (Sn), hafnium
(Hf), zirconium (Zr), manganese (Mn), or magnesium (Mg); an alloy
containing the metal; a nitride of the metal; or a carbide of the
metal. A first electrode layer disposed on an n-type Group-III
nitride compound semiconductor layer is preferably made of titanium
(Ti) or titanium nitride (TiN). A first electrode layer disposed on
a p-type Group-III nitride compound semiconductor layer is
preferably made of nickel (Ni). A second electrode layer is
preferably made of rhodium (Rh)
[0091] In Example 3, the Al layer 32, which serves as a second
electrode layer, is disposed on the Ti layer 31. A layer of a
highly reflective material may be used instead of the Al layer 32.
The highly reflective material layer is preferably made of a single
metal such as silver (Ag) or platinum (Pt), or rhodium (Rh) or an
alloy containing such a metal and may have a multilayer structure.
The highly reflective material layer may have a small thickness of
30 nm or less or a large thickness of 5 .mu.m or more. This case is
within the scope of the present invention.
* * * * *